NEW YORK--(BUSINESS WIRE)--Fitch Ratings has affirmed six classes of FREMF 2012-K20 multifamily mortgage pass-through certificates and Freddie Mac structured pass-through certificates, series 2012-K020 as follows: FREMF 2012-K20 Multifamily Mortgage Pass-Through Certificates --$163.3 million class A-1 at 'AAAsf'; Outlook Stable; --$899.7 million class A-2 at 'AAAsf'; Outlook Stable; --$1.063 billion* class X1 at 'AAAsf'; Outlook Stable; --$1.063 billion* class X2-A at 'AAAsf'; Outlook Stable; -